Electronic article surveillance system

ABSTRACT

A magnetic article surveillance system utilizing microcomputer control and unique time domain and frequency domain information gathering channels whose information is processed by the microcomputer via preselected time domain and frequency domain criteria.

This is a continuation application under 37 CFR 1.62 of priorapplication Ser. No. 353,861, filed May 18, 1989, now abandoned, whichis a division of application No. 091,052, filed Aug. 28, 1987, and nowU.S. Pat. No. 4,859,991.

BACKGROUND OF THE INVENTION

This invention relates to electronic article surveillance systems and,in particular, to electronic article surveilance systems which utilizemagnetic marker for surveillance purposes.

Eectronic article surveillance systems wherein magnet markers areattached to the articles under surveillance are well known in the art.In these systems, an alternating magnetic field is applied to aninterrogation zone via an antenna system. If an article with a suitablemagnetic marker then passes into the zone, the presence of the marker inthe zone causes a perturbation in the field. The field in the zone issensed by a receiver antenna, whose output contains this perturbation. Adetector analyzes the signal from the receiver to assess whether aperturbation to the field has occurred and whether this perturbation isa result of a marker If so, an alarm is activated indicating passage ofthe marker through the zone.

In prior electronic article surveillance systems, the perturbations tothe field created by a magnetic marker have been detected usingfrequency domain or time domain analysis techniques. Typically, whenfrequency domain techniques are employed, the frequency content of thereceived signal is investigated for harmonics of the fundamental (drivefrequency) of the applied field These harmonics are generated as aresult of the non-linear hysteresis characteristics of the magneticmarker. By comparing the relative amplitudes of the detected harmonics,an indication of the frequency spectrum of the signal is obtained. Usingcertain decision criteria this spectrum is then compared with thespectrum expected from a valid marker and a decision as to the presenceof the marker is reached. The use of frequency domain analysistechniques is particularly desirable where noisy conditions may beexpected, but the high Q filters required to isolate the generatedharmonics make the system response time long.

In time domain analysis, the time domain pulse of the received signal isanalyzed with respect to its pulse shape and its time displacementrelative to the fundamental phase of the applied field. With this typeof analysis, the shape of the signal is influenced significantly by theamplitude and group delay characteristics of the filtering used in thedetection process and changes in the received signal due to variationsin the applied drive field. Furthermore, because the analysis utilizesamplitude thresholds which must be set above the ambient noise level,the use of this procedure is most attractive where a highsignal-to-noise ratio is present.

As can be appreciated, whether time domain or frequency domain analysistechniques are utilized in the surveillance system, it is essential thatthe detection process be able to discriminate between perturbations orchanges in the field resulting from valid markers and those fromnon-marker sources. Failure of the system to provide the neededdiscrimination results in false alarms which significantly detract fromthe integrity and usefulness of the system.

The problem of reliably distinguishing between magnetic markerperturbations and extraneous non-marker perturbations is heightened inretail establishments particularly, supermarkets, where metallicfixtures, metal counters, metal shopping carts, noise generators (e.g.,laser scanners, digital scales, credit card and bar code readers,conveyor belts, etc.) and the like proliferate. This equipment creates aharsh electronic environment and may cause perturbations in thedetection system which either mask valid marker signals and/or appear asvalid marker signals. The reliability of the system is, therefore,significantly compromised.

Present surveillance systems have been unable to completely solve thisreliability problem and have suffered from other types of disadvantages.Thus, systems have been designed which are physically very large,causing flow through the interrogation zone to be impeded. Also, thesesystems have utilized relatively strong applied fields. As a result, thefields often extend beyond the interrogation zone, increasing thelikelihood of perturbations from non-marker sources or from markersoutside the interrogation zone. Furthermore, the overrange of thesesystems coupled with electronic noise significantly reduces systemsensitivity. A relatively poor detection rate and an undesirable numberof false alarms, thereby often result.

There has recently been developed a magnetic marker having attributeswhich can alleviate some of the aforementioned problems encountered inthese prior systems. U.S. Pat. No. 4,660,025, entitled "ArticleSurveillance Magnetic Marker Having An Hysteresis Loop With LargeBarkhausen Discontinuities", assigned to the same assignee hereof,discloses a magnetic marker of this type. The marker comprises amagnetic material having retained stress and a magnetic hysteresis loopwhich exhibits large Barkhausen discontinuities. As a result, when themarker is exposed to an applied magnetic field above a threshold value,the marker undergoes a regenerative reversal in its magneticpolarization.

Because this regenerative reversal can be made to occur at a relativelylow threshold value, the applied field required for the marker canadvantageously also be relatively low. Furthermore, the step functionreversal exhibited by the magnetic polarization of the marker results inperturbations of the field which are rich in high harmonics, makingdetection easier and simpler.

The marker of the '025 patent is also advantageous in that it can bedeactivated by a variety of practices as is disclosed in U.S. Pat. No.4,686,516, entitled "Method, System and Appraratus for ArticleSurveillance", and also assigned to the same assignee hereof. Thus byalleviating the retained stress in the marker or by crystallizing aportion of the marker, the marker is easily deactivated so as to be ableto pass through the interrogation zone without producing any alarm.

Another advance which is directed to overcoming the aforementionedproblems of prior systems is disclosed in co-pending U.S. patentapplication Ser. No. 880,138 now U.S. Pat. No. 4,769,631 also assignedto the same assignee hereof and whose disclosure is incorporated hereinby reference. The '138 application discloses a magnetic shield which isto be placed at the side edges of the interrogation zone and which isadapted to reduce the intensity of the magnetic field outside the zone.The shield is also adapted such that the perturbations to the fieldcaused by the shield itself can be easily distinguished and blocked.

As disclosed in the '138 application, to achieve these characteristics,the magnetic material of the shield must have a sufficiently highresistivity, for a given permeability and frequency of the appliedmagnetic field, to provide a skin depth substantially in excess of thethickness of the shield. The magnetic material must also have asaturation flux density greater than the maximum flux density producedin the shield due to the positive and negative peak excursions of theapplied field. Finally, the magnetic material must be such that itresponds to the applied field at the peaks of the field with little orno response near the zero crossings.

As further discussed in the '138 application, shields made from ferritesand pressed powdered iron can provide the aforesaid characteristics andresult in a ratio of peak front field (i.e., field inside the zone) topeak back field (i.e., field behind the shield) of at least ten to one.A particular ferrite which exhibits this 10 to 1 ratio and, in addition,a maximum response at the applied field peaks and a minimum response atthe zero crossings is a ferrite identified commercially as Q5B,manufactured by TDK Corporation of Tokyo, Japan. Similar characteristicsare found in pressed powdered iron, preferably unsintered, having about99 percent iron content with about one percent trace elements includingFeP, H₂, C, Mn, S and perhaps other minute amounts of other elements.

Where the magnetic shield is a laminate of a plurality of thin sheetsglued together and electrically isolated from one another, the skindepth of each such sheet is made substantially greater than the sheetthickness.

The need for minimum response of the shield near the zero crossings ofthe applied field is due to the fact that magnetic markers and, inparticular, the markers of the '025 patent, have a maximum response atthe zero crossings of the applied field and a minimum response at thefield peaks. By selecting the shield magnetic material to have thereverse characteristic, perturbations caused by the shield can be easilydetected and eliminated without interfering with detection of the markerperturbations.

The '138 patent application also teaches, in conjunction with the abovediscussed magnetic shield, an auxiliary shield of electricallyconductive material. This shield is situated behind the magnetic shieldand attenuates, through eddy current losses, perturbations from markersand external noise sources located outside the interrogation zone.

While the above magnetic marker and magnetic shield provide significantimprovements to prior magnetic electronic article surveillance systems,there still exists a need for an overall system having attributes ofsubstantial reliability, compactness and freedom from false alarms.

It is therefore a primary object of the present invention to provide animproved electronic article surveillance system and method.

It is a further object of the present invention to provide an improvedmagnetic electronic article surveillance system and method havingenhanced control functions.

It is a further object of the present invention to provide an improvedmagnetic article surveillance system and method having enhanceddetection practices.

It is yet a further object of the present invention to provide animproved magnetic electronic surveillance system and method havingunique frequency domain and unique time domain detection practices.

It is still a further object of the present invention to provide animproved magnetic electronic article surveillance system and methodwhich is responsive and self adjusting to changes in the surroundingenvironment including changes in noise.

It is a further object of the present invention to provide an improvedmagnetic electronic surveillance system having an antenna system withenhanced properties.

It is yet a further object of the present invention to provide animproved magnetic electronic surveillance system having an antennapedestal with improved properties.

It is yet a further object of the present invention to provide animproved magnetic electronic surveillance system having unique combbandpass filter means.

It is yet a further object of the present invention to provide animproved magnetic electronic surveillance system having a unique combnotch filter means.

It is a further object of the present invention to provide an improvedmagnetic electronic surveillance system as above described with thefurther addition of a magnetic marker as in the '025 patent and a shieldas in the '138 application.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention the above andother objectives are realized in a magnetic electronic articlesurveillance system wherein system operation is under microcomputercontrol. Detector circuitry including frequency and time domain circuitsprocess the received signal from the interrogation zone. Filter meansincluding an unique comb bandpass filter confines the received signal inthe detector circuitry to harmonics of the applied field.

At least two bands of frequencies of the received signal are isolated bythe frequency detection circuitry and the signal content in each banddetermined. At the same time, the time domain circuitry samples thereceive signal to produce a digitized signal. The results of theseoperations are then processed by unique decision routines in themicrocomputer. These routines look to improved valid marker criteria andhave thresholds related to ambient noise and undesired responses fromobjects other than valid markers.

Decision information from the time and frequency domain routines in themicrocomputer is generated for each half cycle of the applied field.This information is used to update respective time domain and frequencydomain counters. Signals for initiating alarm activation are generatedupon the counters reaching preselected counts.

Another routine in the microcomputer processes selected values ofsuccessive digitized signals in accordance with thresholds based uponthe expected characteristics of unwanted high value spuriousperturbations (i.e., electronic noise spikes). A further routine in themicrocomputer looks to changes in the fundamental frequency componentwhich is also isolated by the detection circuitry. The outputs of boththese routines are used to preempt alarm activation by the time andfrequency domain counters.

Noise levels are monitored by the microcomputer upon system initiationand periodically updated, as is the level of the fundamental frequencycomponent. The computer routines are thus updated with these values sothat system is able to dynamically change with changes in theenvironment.

The comb filter means of the system utilizes an integrated circuit delayline provided with feedback to achieve desired comb response. Thisenables filtering with high Q values, while eliminating the need andbulk associated with the discrete capacitors of conventional filters.

The system also incorporates receiver and transmitter antenna arrayswhich are designed to reduce unwanted nulls normally present in theapplied field. The receiver antenna array comprises first and secondupper loops arranged in nested relationship and third and fourth lowerloops also arranged in nested relationship, the first and second loopsforming with the respective third and fourth loops figure eightconfigurations. The loops are connected electrically in series andphased so that the upper loops have the same first phase and the lowerloops the same second phase opposite the first phase. In preferredconfiguration the portion of each loop along the junction in the figureeight configuration is at an angle or slant relative to the horizontal.

The transmitter antenna array also comprises a nested arrangement ofloops. In this arrangement, the current in the inner loop is in the samedirection as that in the outer loop, while the axes of the inner loop isrotated relative to the axes of the outer loop by a preselected amount,preferably, forty five degrees.

The aforesaid antenna arrays of the system are supported in pedestalswhich are adapted to immobilize the arrays and to have substantialphysical integrity. This is accomplished by a process of filling theregion between the walls of the pedestals with foam and placing the unitunder pressure as the foam cures. The resultant rigid foam encapsulatesthe antennas rendering them immobile, while it substantially fills thevoid space between the pedestal walls providing desired strength andstiffness to the pedestal.

It is further contemplated in accordance with the invention that thesystem of the invention utilize a marker as disclosed in the '025 patentand incorporate with the antenna arrays in the pedestal a shield asdisclosed in the '138 application. In such case, the system is furtheradapted to inhibit the aforementioned shield spike pulse frominfluencing system operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 shows a simplified block diagram of a magnetic electronicsurveillance system in accordance with the principles of, the presentinvention;

FIG. 2A-2C show in more detail the block diagram of the system of FIG.1;

FIG. 3 shows waveform diagrams for various signals in the system of FIG.2A-2C;

FIG. 4 shows various system parameters for the system of FIG. 2A-2C andrepresentative values for these parameters;

FIG. 5A-5B illustrate the routines of the microcomputer of the system ofFIG. 2A-2C for time domain and frequency domain signal analysis;

FIG. 6 shows the routine of the microcomputer of the system of FIG.2A-2C for "spike" detection signal analysis;

FIGS. 7 and 8 illustrate the comb bandpass filter of the detectionportion of the system of FIG. 2A-2C;

FIGS. 9, 9A and 9B show the comb notch filter of the detection portionof the system of FIGS. 2A-2C;

FIGS. 10 and 11 illustrate a receiver and transmitter antenna array,respectively, useable with the system of FIG. 2A-2C;

FIG. 12 shows the null zones for a typical antenna array and for theantenna array of FIG. 10;

FIG. 13 illustrates a pedestal arrangement useable for housing theantenna arrays of FIGS. 10 and 11;

FIG. 14 shows a cross-section through one of the pedestals of FIG. 13;and

FIG. 15 shows counters and a peak amplitude stack of the microcomputerof the system of FIG. 2A-2C.

DETAILED DESCRIPTION

In FIG. 1, a magnetic electronic surveillance system 1 is illustrated.As shown, the system 1 includes an AC drive 11 which supplies a highpolarity sine wave AC current and voltage at a fundamental frequency Foto a transmitter antenna 12. The antenna 12, in turn, develops analternating magnetic field at the frequency Fo which propagates into aninterrogation zone 13 through which articles under surveillance, such asarticle 14, must pass.

The article 14 has affixed to it a marker 15 comprised of a magneticmaterial. The marker 15 thus creates perturbations in the magnetic fieldwhich contain harmonics of the fundamental frequency Fo and are coupledor sensed by a receiver antenna 16. The receiver antenna 16 converts theperturbations into an electrical signal which is applied to a controllerinterface and receiver 17. The receiver 17 develops frequency domain andtime domain information from the received signal and through itsinterface makes this information available to the system controller 18.

The controller 18 analyzes this information in accordance with certaindecision criteria. If the decision criteria indicate the presence of avalid marker, the controller addresses an alarm unit 19. The alarm unit19, in turn, activates providing an indication that an article 14 is inthe interrogation zone.

The system controller 18 provides control for the AC drive 11 andreceives transmitter AC current status and voltage information. Also,the controller 18 supplies timing, address and other information to thecontroller interface and receiver 17 as will be described more fullybelow.

In the preferred form of the system 1, the marker 15 is of the typedisclosed in the '025 patent and thus has a hysteresis loop whichexhibits a large Barkhausen discontinuity or step function change influx each time the applied field reverses polarity and exceeds arelatively low threshold value. The perturbation to the field caused bythe marker 15 will thus be in the vicinity of the zero crossings of theapplied field and will be rich in harmonics of the fundamental frequencyFo. Furthermore, the expected marker signal will be a pulse of extremelynarrow width (under 200μsec).

Also, in the preferred form of the system 1, the antenna arrays 12 and16 are provided with shielding as discussed above, which results inso-called "shield spikes" in the received signal which the system 1 mustaccount for and suppress.

While the preferred form of the invention thus utilizes the '025 markerand the shield of the '138 application, the principles of the inventionhave application to other marker and antenna array types. In such case,the system configuration and decision routines would be modified toaccount for the particular characteristics of the particular markerand/or antenna being, used.

FIGS. 2A-2C illustrate in greater detail the system controller 18 andthe controller interface and receiver 17 of FIG. 1. The controller 18comprises a microcomputer 21 which typically might be an 8031microcomputer manufactured by Intel and which effects primary sequentialcontrol over the system operation. The computer 21 communicates with amain program memory EPROM 22 which contains the main programs and thedecision routines for the computer. A second non-volatile memory NOVRAM23 is also provided for storage of operating parameters of the system.The NOVRAM 23 retains these parameters in the event of loss of power tothe system.

A programming interface 24 provides means for externally communicatingwith the microcomputer 21. The interface 24 allows the operatingparameters in the NOVRAM 23 to be set to accommodate the particularon-site conditions in which the system is being used. The interface 24also allows a variety of diagnostics to be carried out to check systemoperation.

The microcomputer 21 provides system status signals to a statusindicator 25. The watchdog circuitry 26 provides the computer 21 with areset pulse upon power up which tells the computer to begin execution ofthe main program. An auxilliary interface 27 is also provided.

An address latch 28 permits the computer 21 to address other componentsin the controller 18 as well as the receiver 17. Analog data from thereceiver 17 is converted to digital data for processing by themicrocomputer 21 through analog-to-digital converter 29. A 12 MHz mainCPU clock 31 provides the primary clock signal for the system.

The CPU clock 31 feeds the computer 21 and a programmable divider 32.The latter develops a plurality of further clock signals synchronized tothe main clock for application to the microcomputer and other componentsof the system.

The controller 18 also includes a drive channel 30 for establishing thedrive current for the transmitter 12. The channel 30 includes low passfilter 33, level adjuster multiplexer 34 and buffer amplifier 35.

FIGS. 2A-2C also shows a power amplifier 79 of the AC drive 11 and acurrent sensor 81 associated with the transmitter antenna 12. Audioalarm 82, visual alarm 83, event counter 84 and alarm interface circuit85 of alarm unit 19 are likewise shown.

As discussed above, program interface 24 is used to address themicrocomputer 21 in order to set in the NOVRAM 23 the particularoperating parameters desired of the system. These parameters will, inmost cases, be dependant on the environment in which the system 1 is tobe used. FIG. 4 shows representative parameters programmable into theNOVRAM 23 and illustrative values for these parameters.

The NOVRAM 23 is also provided with an initial set of parameter values,so-called default values, which are preset for a nominal environment.These parameters can be used to govern system operation, in the eventthe environmental conditions are nominal.

Once the system parameters are set, the microcomputer 21 proceeds to setthe transmitter antenna current through the drive channel 30. For thispurpose, a pulse signal at the desired drive frequency Fo is generatedby the microcomputer 21. This pulse signal is developed from a furtherpulse signal SSB1 which is at twice the desired frequency (2Fo) andwhich is developed by programmable divider 32 from the 12 MHz clocksignal.

The pulse signal at frequency Fo is passed by the microcomputer 21through low pass filter 33 and converted into an AC sine wave signal.The resultant AC signal passes through the level adjuster 34 which hasbeen addressed by the microcomputer to set an initial current level andis passed through buffer amplifier 35 to the power amplifier 79 of thetransmitter AC drive 11. The output of the amplifier 79 drives thetransmitter antenna 12 at the frequency Fo and a magnetic field at thisfrequency is established in the interrogation zone 13.

Current sensor 81 senses the current in the transmitter antenna coilsand delivers its output to port TXI of an analog multiplexer 75 in thereceiver circuit 17. This port is addressed by the microcomputer 21through address latch 28 and address port ADDR of multiplexer 78 and thetransmitter current is read via analog-to-digital converter 29.

The microcomputer 21 then compares this current value to the presetcurrent value stored in NOVRAM 23 and if there is a difference addressesthe level adjuster 24 through address latch 28 to cause the current tochange toward the preset value. This process continues until the currentlevel is set to the preset level.

Once the current level is set, the computer 21 adjusts the phase of thedrive current via the Fo signal. This adjustment is such that the peaksof the drive current are at the zero level intervals of the SSB1 signaland the zero crossings are at the logic one level intervals of the SSB1signal. This relationship is illustrated by the waveforms 301 and 302 ofFIG. 3 and, as can be appreciated, allows the signal SSB1 to providesignal blanking during peak amplitudes of the transmitter current andsignal passage near the zero crossing intervals. The signal SSB1 can,therefore, be used to blank the shield spike (shown in waveform 303 inFIG. 3) which develops in the system as a result of use of the shielddisclosed in '138 application. This will be discussed further below.

As shown in FIG. 2A-2C, the system 1 is also further adapted to ensurethat the transmit antenna array is operating at resonance. Thus, themultiplexer 75 of the system, in addition to receiving transmittercurrent information from current sensor 81 at port TXI also receives atport TXV transmitter voltage data from the output of the antenna poweramplifier 79. The microcomputer 21, at initialization, reads and storesthe transmitter current and voltage information at multiplexer ports TXIand TXV.

The microcomputer also compares the phases of these signals to seewhether a phase difference exists. If the phases differ, themicrocomputer recognizes a non-resonance condition of the transmitterantenna 12 and adjusts the transmit frequency to bring the transmitcurrent into phase with the transmit voltage so that the transmitantenna operates at resonance.

Having set the phase and level of the transmit current, themicrocomputer 21 then addresses, via address latch 28 and address portADDR, the ports PFo, PFB1, PFB2 and PFB3 of the multiplexer 75. At theserespective ports, the receiver 17 provides the fundamental componentlevel of the received signal and the the received signal energy levelsin three preselected frequency bands FB1, FB2 and FB3. The microcomputer21 treats the levels in the latter three bands as ambient noise levels.Averages of these levels and the fundamental level are stored by themicrocomputer for future use in the system operation.

The computer now begins its surveillance operation. This operation isconducted repetitively by the computer every half cycle of the transmitcurrent, which half-cycle is referred to as a "frame". During a firstinterval or this half cycle, referred to as the "marker window" intervalthe microcomputer causes the receiver 17 to gather frequency and timedomain information from the received signal which contains anyperturbations occurring to the field in the interrogation zone. Themicrocomputer reads this information from the receiver 17 and during theremaining interval of the frame (the "processing interval"), via itsdecision routines, evaluates whether the information is indicative of avalid marker in the zone.

In the receiver 17, the received signal is branched through a firstchannel A comprised of fundamental detector 50. The detector 50extracts, via a bandpass filter, the fundamental frequency component(i.e., the component at Fo) of the signal. This level is then madeavailable to the multiplexer 75 at the port PFo for subsequent analysisby the microcomputer.

The received signal is also branched through a second channel B whereinthe notch filters 51 and 52 remove, respectively, the frequencycomponent of the signal at Fo and at the frequency of the power line tothe system (i.e., frequencies in a range of 50-60 Hz). Extraction ofthese frequency components removes signal content which might otherwiseresult in false marker indications.

The resultant signal from filter 52 is then amplified in preamplier 53and the amplified signal passed in sequence through high pass and lowpass filters 54 and 55. These filters effectively isolate the frequencyband where harmonics of interest are expected for the markers used withthe system. The particular frequency range indicated in FIGS. 2A-2C isfor the preferred marker of the system (the '025 patent marker) which iscapable of providing substantial harmonic content over the indicatedrange of 1-8 KHz. By limiting the frequency band of the signal to thisrelatively high range, the effects of non-marker disturbances and noiseon the detection process are minimized.

After filtering, the signal is then passed through a pre-emphasiscircuit 56 which is utilized to compensate for the rolloff of the signalcaused by subsequent passage through comb bandpass filter 58. Fromcircuit 56, the signal amplitude is limited in limiter 57 and thenconveyed to the latter comb bandpass filter 58 which, as will bediscussed below, comprises a digital delay line which is clocked at amultiple (1024) of the fundamental frequency Fo.

The filter 58 has narrow passbands at the fundamental Fo and itsharmonics and thus passes only harmonics of Fo. The filter thereforefurther confines the signal to only those frequency components expectedto result from the system marker. The design of the filter 58 also addsto the compactness of the system, since it does not employ discretebulky capacitors.

After passage through filter 58, sampling noise caused by the combbandpass filter is removed by low pass filter 59. The resultant signalfrom the filter 59 is now sufficiently frequency and amplitudeconditioned for passage through the time domain and frequency domainchannels C and D of the receiver.

In the frequency domain channel D, the signal is first passed throughshield spike blanker 61 which is controlled by the signal SSB1 so as tosuppress the signal content during the zero or blanking interval of thesignal SSB1 and pass the signal during the non-blanking interval of thesignal. As discussed above, this blanking occurs at the peak values ofthe transmit current which is where the shield spike from the antennashield occurs. Blanking is necessary because the shield spike isfrequency coherent and cannot be filtered out. These spikes are thusremoved from the signal by the blanker 61.

After blanking, the resultant signal is branched through a plurality offrequency domain sub-channels. Each of these sub-channels isolates aparticular frequency band of the signal and determines the energy inthis band. These energies are then used by the microcomputer in itsfrequency domain decision routine in evaluating for marker presence.

The number of frequency domain sub-channels is selected so as to ensurethat at least two frequency bands are obtained which are reasonablycertain to have frequency content expected from a marker. In thepreferred form of the invention, three sub-channels D1, D2, D3,corresponding to low, mid, and high frequency bands FB1, FB2, and FB3are utilized.

More particularly, the frequency bands FB1, FB2, and FB3 are isolated byrespective low, mid and high range bandpass filters 62, 63, and 64, andfull wave rectified in respective circuits 65, 66, and 67. Sychronousintegrators 68, 69, and 71 then integrate the rectified signals, therebydeveloping DC values representative of the energy in each of the bandsFB1, FB2, and FB3. These DC values appear at the respective ports PFB1,PFB2, and PFB3 of multiplexer 75 for subsequent processing bymicrocomputer 21.

The operation of the integrators 68, 69, and 71 is synchronized with themicrocomputer operation by a second synchronizing signal SSB2 which isdirected to the integrators via level shifter 76. The signal SSB2 is atthe same frequency (2Fo) as the signal SSB1, but is slightly shifted inphase to account for the signal delay introduced by the filters 62-64.It should be noted also that the gains of the filters 62-64 are selectedso that they normalize the outputs of the integrators 68, 69, and 71based upon a preselected characteristic representative of a responseexpected from system markers. Thus with a marker in the zone 13, theoutputs of integrators 68, 69, and 71 will be equal or ascending levels.

For the frequency band of interest in the present illustrative case, thebands FB1, FB2, and FB3 are centered about frequencies 1.5, 2.5, and 3.5KHz. Furthermore, the bandwidth of each band is 600 Hz.

Turning now to the time domain channel C, this channel also receives theoutput from low pass filter 59 and passes it through a high pass filter72. The lower cutoff of high pass filter 72 is selected so as to passsufficient high harmonics of the signal to be able to obtain a goodpicture of the expected changes in a marker pulse. By placing the cutoffat the approximate center of the high bandpass sub-channel D3 (at 3.5KHz for the above example), sufficient signal content is reasonablyassurred.

The signal from filter 72 is rectified in full wave rectifier 73 andsampled in sample and hold circuit 74 to obtain a digitized version ofthe signal. This digitized signal is made available to the microcomputer21 through a further port PTD of multiplexer 75.

The sampling interval and the timing of the sample and hold circuit 74are controlled by the microcomputer 21 through the sequencing logic 77.During each sampling interval the sample and hold circuit samples thesignal and retains the previous sample. The microcomputer controls thiscircuit so that as the circuit aquires a new sample, the microcomputerreads and stores the previous sample contained in the holding circuit.

At the end of the marker window interval, the microcomputer 21 has thusstored in its memory a digitized form of the filtered and rectifiedreceived signal. At this time, the microcomputer accesses the linesPFB1, PFB2, and PFB3 of the multiplexer 75 and reads and stores the DClevels for the frequency bands FB1, FB2, and FB3.

The microcomputer then initiates its processing phase wherein itanalyzes this time and frequency domain information via its decisionroutines. Based upon the results of these routines the microcomputerupdates time domain and frequency domain counters 21a and 21b (see FIG.15) and checks the count in each counter. Only when the counters areboth at preselected counts indicative of marker presence over a numberof frames does the microcomputer reach a decision that a marker ispresent. This decision results in the microcomputer 21 activating thealarms 82 and 83 via the alarm interface 85, unless the alarm decisionis preempted as a result of further spike detection and fundamentalevaluation routines described herein.

FIGS. 5A and 5B show flow diagrams of illustrative time domain andfrequency domain routines 100 and 200 of the computer 21. These routinesare based upon experimental frequency and time domain data of non-markerobjects, such as shopping carts, and similar data for markers of the'025 type. They are also based upon experimental data as to noise levelsand minimum detectable signal levels. Finally, they assume that a framecontains 64 equal intervals and that the marker window contains 24intervals each of which has associated with it one sample of the storeddigitized signal.

The time domain routine is initiated first and when completed thefrequency domain routine proceeds. In the present preferred case bothroutines are always carried out for each frame of operation.

In steps 101 and 102, the time domain routine 100 determines the peaksample value Ps of the 24 samples of the digitized signal in the markerwindow interval. By examining the signal in this interval only, anycontent due to antenna shielding is eliminated and the computereffectively blanks shield spikes.

In step 101 the peak sample Ps is compared to a 0.2V threshold and ifless than this threshold the routine ends, the time domain counter 21ais decremented and the frequency domain routine started. If the peaksample is above the 0.2V threshold, the step 102 is instituted and thelevel at half the peak sample (the 6dB threshold) is compared to theambient noise level previously determined and stored by themicrocomputer. If the 6dB threshold is less than the noise level, theroutine ends, the time domain counter 21a is decremented and thefrequency domain routine instituted.

If the 6dB threshold is greater than the noise level, step 103 isinitiated and a determination is made as to whether more than 6 samplesof the 24 samples in the marker window interval are above the noiselevel. If more than six samples are above the noise level, the routineends, both the time domain and frequency domain counters are set tozero, and the frequency domain routine is started. If not, then step 104begins.

In step 104, the position in the frame of the peak sample Ps is comparedto the position in the frame of the peak sample two frames previously.These positions are referenced to phase positions PH1 and PH2 of thesamples in their respective frames and if these phases are different bymore than 2.8 degrees (i.e., one sample interval), the routine ends,both the time domain and frequency domain counters are returned to zeroand the frequency domain routine begins.

If the difference between these phases is less than 2.8 degrees, thestep 105 is instituted and a determination is made as to whether thenumber of samples, above the 6dB threshold is greater than 3. If yes,the routine ends the time domain counter is decremented by 1 and thefrequency domain routine is started. If no, the routine proceeds to step106.

In step 106, all the samples above the 6dB threshold are compared to seewhether they are within 2 sample intervals of each other. If no, theroutine ends, the time domain counter is decremented by 1 and thefrequency domain routine started. If yes, the time domain counter isincremented by 2 and the step 107 instituted.

In step 107, it is determined whether the number of samples above the10dB threshold (i.e., 1/3 the peak sample value Ps) is greater than 4.If yes, the routine ends, and the frequency domain routine instituted.If no, the routine proceeds to step 108.

In this step, it is determined whether all the samples above the 10dBthreshold are within 3 samples of each other. If yes, the time domaincounter is incremented by 1, the routine ends and the frequency domainroutine instituted. If no, the routine ends and the frequency domainroutine instituted.

In summary, the time domain routine 100 examines the digitized signalsamples to first determine whether the signal level is within theacceptable accuracy levels of the receiver equipment (step 101) and thenwithin an acceptable signal-to-noise ratio (step 102). If either ofthese conditions is not met, the ability to make a marker determinationis not present and, therefore, the counter 21a is decremented by 1.

If these conditions are met, the samples are then examined to assess therelative pulse width of the signal (step 103). Thus, if a relativelylarge number of samples (6 in the present case) are above the noiselevel, this indicates either an overly large pulse width not expected ofa valid marker or too much noise during the marker window to make avalid marker decision. In either case, this is considered a seriousfailure, making detection of the presence of a marker highly unlikely,and both the time and frequency domain counters are returned to zero.

If the pulse width test is met, a phase test (104) is then carried out.This test looks to the position of the peak sample in relation to thepeak sample two frames previous. In particular, the difference in thepositions of these samples in their respective frames is required to bewithin one sample interval. This requirement is based, in part, on thefact that a valid marker should provide a signal at substantially thesame point in each frame, while this would not be expected of the pulsesfrom non-marker objects. The specific position error of one sampleinterval is based on sample and hold circuitry phase error and markerlocation in the field.

Furthermore, by comparing the peak samples in every other frame theinfluence of the earth's magnetic field on the analysis is removed. Thisis so because, depending upon the orientation of a marker in the zone 13with respect to the applied field and the earth's field, theperturbations to the field caused by the marker and the resulting markersignal will not occur in the same position in the tag window insuccessive frames (i.e., alternate polarities of the drive). Moreover,for weak marker drive the signal may be absent from every other frame.Thus, the testing of alternate frames eliminates erroneous decisionswhich might occur due to the earth's magnetic field.

Failure of this phase test is also considered to establish that thepresence of a marker is highly unlikely. Its failure is treated as wasthe failure of the pulse width test and both counters 21a and 21b arereset to zero.

If the phase test (104) is passed, a refined pulse width test (steps 105and 106) must be carried out. If the samples above the 6dB thresholdexceed three, this test fails. Moreover, if those samples above the 6dBthreshold are not spaced within two sample intervals of each other, thetest also fails. These failures, while indicative of a pulse widthgreater than expected from a valid marker, are not such as to make thelikelihood of a marker entirely remote. The counter 21a is thus onlydecremented by 1 to indicate this condition.

If this second pulse width test is satisfied, there is a good likelihoodthat there is a marker present and the counter 21a is incremented by 2to register this fact.

The final test (107 and 108) constitutes a further refined pulse widthexamination and if passed establishes a higher degree of confidence thata marker is present. If failed, however, this test does not detract fromthe earlier finding of the likelihood of marker presence. Passage ofthis test further increments the counter 21b by one, but its failuredoes not change the counter.

At the end of time domain routine 100, the frequency domain routine 200is initiated. The frequency domain routine looks at the energy in thefrequency bands FB1, FB2, and FB3 which are represented by the DC levelsfrom integrators 68, 69, and 71. It then determines whether the slope ofthe frequency spectrum of the received signal is as expected of a validmarker and not as expected of non-marker objects.

As explained above, the filters 62, 63, and 64 have been designed sothat the expected integrated outputs of the bands FB1, FB2, and FB3 formarkers would result in equal or ascending DC values. Thus, thefrequency domain routine checks for this increase in DC level.

More particularly, the first step in the frequency domain routine is todetermine whether the 6dB threshold value for the DC level of the highband FB3 is less than the noise level. If yes, the test fails, thefrequency counter 21b is decremented and the routine ends. If n, theroutine continues to step 202 where the DC level of frequency band FB1is checked to determine whether it exceeds a predetermined thresholdvalue shown at 3 volts. This predetermined value is based on systemcontraints which are set to prevent the associated DC level for the bandfrom exceeding this level for a valid marker under normal systemoperation. If the predetermined level is exceeded, the likelihood isthat a valid marker is not present and the counter 21b is decrementedand the routine therefore ends.

If the level is not exceeded, steps 203 and 204 are initiated, and theDC level of the band FB1 is compared to the DC level of the band FB2 andthe DC level of the band FB2 is compared to the DC level of the bandFB3. If either the band FB1 DC level is greater than the band FB2 DClevel or the band FB2 DC level is greater than the band FB3 DC level,the test fails. The counter 21b is then decremented and the routineends.

If the band FB1 DC level is less than the band FB2 DC level and the bandFB2 DC level is less than the band FB3 DC level, the presence of a validmarker is likely and the counter 21b is incremented by 2 and the routineends.

As discussed above, after the end of the frequency domain routine, thecounters 21a and 21b are checked to determine whether their countsexceed the set preselected values indicative of the presence of a validmarker. In the illustrative system, the counters 21a and 21b are set soas to count up to a maximum of 12 and can count down to a minimum ofzero. Furthermore, a count of 10 in each counter results in a decisionof valid marker presence.

If the checking operation of the counters 21a and 21b does not result inthe preselected counts in both counters, the computer 21 institutes theinformation gathering phase for the next frame of the applied field andthe above procedure is repeated.

If the checking operation of the microcomputer does indicate arrival atthe preselected counts, an alarm signal will be initiated provided it isnot preempted by two further test routines of the computer.

One of these test routines is a spike detection routine to test thelikelihood that the received signal might have resulted from electricalnoise spikes and the other a fundamental frequency component test todetermine whether the component of the receiver signal at thefundamental frequency has not exceeded a predetermined thresholdassociated with large metal objects in the zone 13.

The spike detection routine is shown in FIG. 6. As previously indicated,it is used to discriminate valid marker signals from electrical noisespikes arrising from power line transients from electrical discharge,switch closures, motor contactor noise, fluorescent and neon lights andso forth. Such noise sources generate transients in the form ofelectrical impulses or "spikes" which may not be discriminated by thetime and frequency domain routines 100 and 200, because these spikesexhibit many characteristics similar to the type of magnetic marker thesystem 1 is set to respond to.

Specifically, noise spikes can exhibit the following features: (a)fundamentally, spikes are an impulse response, as is the signal from avalid marker, therefore, the time domain response from a spike may bequite similar to that of a valid marker; (b) spikes frequently have fastrise and fall times, and therefore produce a frequency spectrum similarto a valid marker; (c) depending on the source, spikes can be of anamplitude which will not be automatically rejected by the system; (d)because of the "echo" effect of the comb bandpass filter 58, a singlespike can appear in multiple marker windows; and (e in general, many ofthe characteristics of noise spikes are similar enough to those of avalid marker that the filtering portions of the receiver 17, have littleeffect in attenuating them.

The spike detection routine depends upon the maintenance of a memoryarray called the peak amplitude stack (see FIG. 15). The stack locationscontain peak values of the sample and hold circuit 74 during successive"tag window" intervals. On a continuing basis, location 1 is updatedwith the current frame peak value, the previous contents being pushedinto location .0.. The first two stack locations thus contain a recordof the marker or marker-like impulse.

Once time domain counter 21a reaches the preselected count indicative ofa valid marker, the operation of the peak amplitude stack changes. Now,peak amplitudes of successive frames are placed in the stack fromlocations 2 through 15. When the frequency domain counter now alsoreaches its preselected value and computer 21 is about to address thealarm, the computer branches to the spike detection routine 500. Itshould be noted that the stack is not necessarily full at this point.The stack stops loading the instant the time and frequency domaincounters are at their preselected counts.

In step 501 the microcomputer 21 examines the contents of the stack fromlocation 2 to the location indicated by the stack pointer to determinethe largest peak value. Once this is found, the microcomputer determineswhether the peak occurred in an ODD or EVEN frame. If the maximum peakoccurred in an odd frame, then the microcomputer advances to step 503where an index called the Frame Pointer is set to stack position 1, ifnot, the microcomputer advances to step 502 where the Frame Pointer isset to stack position .0.. The reason for this action relates to thephysics of magnetic system. Due to the influence of the earth's magneticfield on the system 1, two things can happen: (1) depending on theorientation of the system 1 antennas, the earth's field may alternatelyaid and oppose the system drive field as the drive field switchespolarity, causing the marker signal in alternate windows to be ofdifferent amplitudes; (2) the earth's field acts as a DC bias on thematerial of the marker and depending on the marker orientation, themarker will "switch" and generate at different points in time insuccessive windows. In extreme cases a true marker signal may onlyappear in every other window.

A third reason for this ODD/EVEN approach in the spike detection routineis that a noise spike input wil be echoed by the Comb Bandpass Filter58, every other frame.

Once this ODD or EVEN condition has been determined the microcomputer 21advances to step 504 where only those appropriate stack frames areconsidered. For the present it will be asssumed that the EVEN casepertains, and the Stack Pointer stopped in frame 13.

In step 504 the rise time of the peak signal is determined. In mostcases, the rise time of a spike will be faster then that of a marker.The Frame Pointer is now set to .0.. An initial slope determination ismade by dividing the value in the frame 2 by the valve in frame .0.. Themicrocomputer then advances to step 505 where an evaluation of thisslope is made. If this ratio or slope is equal to or greater than 4 thenthe signal is considered a spike and the microcomputer advances to step517 where the spike routine is exited, otherwise the microcomputerproceeds to step 506.

In step 506 if X is .0. or 1 then the microcomputer advances to step 515where the Frame Pointer in incremented. A spike will frequently appearin one frame but not in the previous one, while a marker entering thesystem field will show a gradual buildup in amplitude from frame toframe. This test determines whether or not there was a signal in frame.0. or 1 that did not quite qualify and pass the time domain criteria.

If the frame pointer in step 506 is not .0. or 1 then the microcomputeradvances to step 507 where a test is made to determine whether the peakin frame X is equal to or greater than 6dB above the noise level (twiceas large). If not, the signal is too small to make a decision and themicrocomputer proceeds to step 515 where the Frame Pointer isincremented by two. If the test passes the microcomputer advances tostep 508 where a test is made to determine whether the peak in frame Xis equal to or greater than 10dB above the noise level (three times aslarge).

If not, the microcomputer proceeds to step 509 where the slope is againtested. If the slope is equal to or greater than 1 then a "possiblemarker" counter is incremented in step 510. If the slope is not greaterthan 1 then the counter is decremented in step 512. If the S/N ratio isequal to or greater than 10dB, the microcomputer proceeds to step 511where the slope is again tested. If the slope is equal to or greaterthan .0..85 then the possible marker counter is incremented in step 510.If not, it is decremented in step 512 and proceeds to step 513.

In step 513 the possible marker counter is now checked and if the countis 2 then the microcomputer advances to step 514 where at least fivesuccessive marker signals have been received which have the proper rateof amplitude increase, and the routine is exited with the decision thatthe signals come from the valid marker. If the possible marker count isless than 2, then the microcomputer advances to step 515 where the FramePointer is incremented and proceeds to step 516.

In step 516 the new Frame Pointer location is compared to the StackPointer (in this case 13). If the Frame Pointer is beyond the StackPointer, then there are no more slopes to compare, it is assumed thepeak values were spikes, the routine is exited and no alarm is generated(step 517). If the Frame Pointer is less than the Stack Pointer, thenthere is more information to process and a new slope is calculated instep 504.

Once the spike detection routine has been completed, if the decision wasthat a spike was present the counters 21a and 21b are reset to zero, noalarm is activated and the system 1 continues surveillance. If thedecision was that a marker is present, the computer makes one final testbefore alarming. In this test the computer 21 compares the fundamentalfrequency component of the received signal read from multiplexer portPFo with its stored value and if the fundamental has changed beyond apredetermined threshold value, the decision is made that there is not avalid marker present. The computer 21 thus does not alarm the system andthe counters 21a and 21b are returned to zero and surveillancecontinues. If the threshold is not exceeded the alarm activatesindicating the presence of a marker in zone 13.

As indicated above, the microcomputer 21 upon initiating operation,makes a determination of the noise levels in the frequency bands FB1,FB2, and FB3 and uses these levels to establish an average noise levelfor the system 1 and for use in its decision routines. The microcomputerupdates this noise level by periodically designating frames as noiseupdate frames. During these frames, the microcomputer treats the DClevels, developed from the sub-channels as indicative of the noise levelin the system and averages these levels to obtain an average noisevalue.

After a preselected number of noise update frames have been passed, themicrocomputer averages the stored average noise values. This average isthen treated by the microcomputer as the new ambient noise level andused in the subsequent decision making routines. The system 1 thusdynamically updates itself to accommodate for changing ambientconditions that might otherwise detract from the ability of the systemto effect reliable marker detection.

In FIG. 7, a block diagram of the comb band pass filter 58 isillustrated. The filter comprises an integrated circuit, delay line 601,input and output ports 602 and 603 and a clock drive port 604. Theoutput port of the delay line 603 is fed back to the input port 602 viafeedback path 605 comprised of resistors R1 and R2.

A clock source 606 supplies clock signals to the port 604 of the delayline 601. The delay line 601 may typically be a Reticon R5107 integratedcircuit. In such case, it contains 512 delay stages or monolithiccapacitors and requires 2 clock cycles to shift from stage to stage.

The filter 58 in the subject configuration will pass harmonics of afundamental frequency Fo if the clock rate is set at a value equal tothe number of delay states times the frequency Fo times the cycles ofthe clock required to shift from one stage to the next. Thus for a 512stage line requiring 2 cycles to shift and the frequency Fo of thesystem fundamental, a clock rate of 1024Fo is required, which is thesignal applied to the filter 58 in FIG. 2 by level adjuster 76. In thefilter configuration of FIG. 7, the Q of the filter is adjusted byadjusting the resistor R₁.

FIG. 8 illustrates an actual circuit configuration of the filter 58. Inthis case, the comb bandpass circuit is used to emphasize multiples of73 Hz. R220, R221, and U203A form an inverting buffer amplifier, whichis used to drive the comb bandpass circuit. The parallel combination ofR222, R223, R224 correspond to R1 in FIG. 7, while R225 corresponds toR2. The three parallel resistors allow the Q of the circuit to beoptimized, simply by clipping and removing the appropriate resistor(s).For most installations, all three parts will remain in the circuit.

C208, R226, and U203B allow the signal to be fed into the delay line ICU201. C208 serves as a DC block, with U203B as a unity gain, high inputimpedance, non-inverting buffer. The combination of high impedance andfairly low capacitance results in low phase delay in the DC block. R226provides a DC path to ground for U203B.

As stated before, U201 is a delay line IC. The IC used in thisimplementation is the Reticon R5107. This IC contains 512 monolithiccapacitors, and thus is clocked at a rate of 74.752 KHz (1024×73 Hz).

The remaining circuit component is U203C. As the Reticon R5107 has lowoutput drive capability, a buffer must be used on its output. U203C isconfigured as a non-inverting buffer, and is able to supply enoughoutput drive so that R225 is not too great a load.

In the discussion of system 1 above, it was pointed out that the channelB is provided with a notch filter which removes the power line frequencycomponent from the received signal before the signal continues into thesubsequent channels. In environments where the line frequency noisecomponents are strong, the system can be further adapted to suppress notonly the line frequency, but also all its harmonics. In particular, isshown in FIG. 8, a comb notch filter 58A having rejection bands at theline frequency and harmonics of the line frequency may be switched intothe channel of the receiver preceding the comb bandpass filter 58 inorder to remove the harmonics of the line frequency.

As indicated above, use of the notch comb filter 58A is particularlyattractive in environments where the line interference is of significantmagnitude. Where the magnitude is not significant, the filter may beswitched out of the channel to avoid possible addition of noise to thesystem.

FIG. 9 shows a comb notch filter in accordance with the invention. Thenotch filter 701 comprises a comb bandpass filter 702 including adigital delay line 703. Signal is fed from the output port 703b of thedelay line back to its input port 703a via a buffer 707, resistor R₂,capacitor C₁, and resistor R₃. The input signal is coupled to the inputport 703a through an inverting buffer 708, resistor R₁, capacitor C₁ andresistor R₃.

The input signal and input to port 703a are coupled to a summing circuit704. Because the latter inputs are out of phase due to the invertingbuffer 708, the power line frequency and its harmonics in the input aresubtracted and effectively removed from the input. A comb notchcharacteristic thereby results.

The clock signal for the delay line 703 is derived from a power linereference frequency which is passed through a squaring circuit 707, aphase lock loop 708 which operates at 2n, where n is the number of delayline stages, times the power line frequency. A divider 611 performs a 2ndivision of the phase lock loop output to provide a proper reference forthe loop.

FIG. 9A-9B show an actual circuit embodiment of notch comb filter 601for the system 1. The signal squaring circuit is made up of R25-29, C27,and U8A. R25 and R27 function as a voltage divider to insure the inputpower line sine wave amplitude does not exceed the safe input level ofU8A. R25, along with R27, also low pass filter the input signal. R28 andR29 serve as gain setting resistors, which insure that the circuitoutput is a rectangular pulse train. This high gain causes the pulsetrain to move quickly between its positive and negative levels. R26supplies hysteresis to the circuit, which also serves to shorten thistransition time. This is necessary as the phase locked loop circuitwhich follows requires short transition times.

The phase locked loop is composed of U2, U3, C17, C18, R22, and R33. Thephase locked loop is used as a frequency multiplexer to supply thenecessary clock signal to the delay line IC U1. The phase locked loopvoltage controlled oscillator (VCO) operates at a frequency that is 2048times the power line frequency. A 2048 divider is in the loop, so theVCO can be phased locked to the power line signal.

As the shield spike is an unwanted signal that would add noise to theoutput of the comb notch filter, U5 an U6 generate the blanker controlsignal from SSB1 and 64FO This signal operates electronic switch U4C sothat the shield spike is not passed by U7A. Besides serving as blanker,U7A doubles as an inverting buffer at the input of the comb band passpart of the comb notch circuit.

The comb bandpass section is composed of U1, U7B U7C, U7D, R3, R4,R7-10, R21, and C13. The operation or this portion of the circuit isidentical to that described earlier for the comb bandpass filter, withthe exceptions that the delay line IC is the Retican R5108 and that theclock frequency is 2048 times the filter fundamental frequency.

The summing circuit is composed of R5, R6, and U8B. R5 and R6 set theproportion in which the comb notched signal and the input signal are tobe added together. They are not added in equal proportion because of theblanking which was done on the comb notched signal. The signals subtractbecause the comb notched signal was inverted by U7A.

After the summing circuit, a low pass filter is employed to suppress anyswitching noise generated by U1. The filter cutoff frequency is 8 KHz,so that all of the marker harmonics of interest are passed. After thefilter, an amplifier is employed to set the remaining signal levels tothe same amplitude they would have if the filter was not employed.

In accordance with a further a aspect of the invention, the transmit andreceive antenna arrays of the system 1 are designed to provide reductionin the usual nulls found in the field in the zone 13.

FIG. 10 shows the receiver antenna array 90 adapted in this manner. Theantenna array comprises first and second upper loops 91 and 92 which arein nested relationship and first and second lower loops 93 and 94 whichare also in nested relationship. The nesting is such that the center C₂of loop 92 is offset from the center C₁ of loop 91 and the center C₄ ofthe, loop 94 is offset from the center C₃ of the loop 93. The upper loop91 is further arranged in figure eight relationship with the lower loop93 and the upper loop 92 is arranged in figure eight relationship withlower loop 94.

The coplanar loops 91-94 are all wound in a clockwise sense as shown bythe arrows and are all connected electrically in series. Furthermore,the loops are phased so that the upper loops 91 and 92 are of the samefirst phase and the loops 93 and 94 are of the same second phase, thefirst and second phases being opposite to each other.

In the preferred form of the array 90, the loop sections at theinterface region of the figure eight configurations, i.e., 91A, 92A,93A, and 94A, are at a slant relative to the horizontal axis. A typicalvalue for the slant angle is 20°. In the preferred case shown, the outerloops 91 and 93 are also mirror images of each other as are the innerloops 92 and 94.

FIG. 12 shows pictorally in I the null zones of a receiver antenna arrayhaving a figure eight configuration. As can be seen, the antennaexhibits nulls along the vertical axis of the array in three distinctbands 11-1, 11-2, and 11-3. FIG. 12 also shows in II the null zones ofthe improved antenna array 90 of the invention.

As can be seen, the null zones have been reduced to only one zone 11-1,which is at an angle or slant relative to the horizontal. This nullzone, however, because of its slant will now advantageously couple withvertically oriented markers along the x-axis so that its effect issignificantly minimized.

FIG. 11 shows an improved transmitter antenna array 201 for the system 1and particularly adapted for use with the receiver antenna of FIG. 10.The antenna array 201 comprises a first single loop 202 in which isnested a second coplanar loop 203. The main axis of the second loop 203is rotated relative to the main axis (y-axis) of the loop 202. In thiscase, the loops are both wound clockwise and are phased the same. Asshown in FIG. 11, both loops 202 and 203 are concentric.

The presence of the inner loop 203 of the antenna array 201 results in afield in the y direction along the x-axis of the loop 202 which wouldnot be the case if the loop 203 were absent. In this regard, asabove-mentioned, the receiver antenna of FIG. 10 is now able to couplewith fields along the horizontal or x-axis so that antennas in thesystem 1 of the invention now enable detection of markers passingthrough the zone 13 along the horizontal direction.

In a further, aspect of the system 1, pedestals of unique constructionare utilized to house the transmitter and antenna arrays of thesystem 1. FIG. 13 illustrates the pedestal construction and FIG. 14shows a cross-section through the pedestals.

As shown, the pedestals 301 comprises facing walls or shells 302 and 303made preferably of plastic and joined to form a clamshell type cavityfor housing transmitter and receiver antenna arrays depicted by coils304 and 305. The arrays are mounted between stiffening bars 306 whichare situated at the lateral ends of the cavity.

Filling cavity void space and surrounding the coils 304 and 305 is .arigid, high density foam 307, such as a urethane. The high density,rigid foam 307 immobilizes the antenna coils and imparts considerablestrength and stiffness to the completed pedestal.

The edges of the shells 302 and 303 are formed into hooks 308 which areadapted to receive an extruded edge strip or bumper 309. A support base311 is further provided to fasten the pedestal to the floor.

The pedestals are formed by first mounting the internal componentsincluding array coils 304 and 305 in one of the shells 302. Thecomponents are then tacked in place with an adhesive such as a hot melt.Both shells 302 and 303 are then filled with a predetermined amount ofchemically reactive foam (such as polyurethane, consisting of variouscombinations of polyolesocyanate based foams) of desired density. Theshells are then closed and placed into a platen press for a short termcure (5 to 10 minutes), resulting in a rigid, compact structure. Theremaining portions of the pedestals are then added to complete thepedestal.

In all cases, it is understood that above-identified arrangements aremerely illustrative of the many possible specific imbodiments whichrepresent applications of the present invention. Numerous and variedother arrangements can readily be devised in accordance with theprinciples of the invention without departing of the spirit and scope ofthe invention. Thus, for example, the system 2 can be provided with afurther multiplexer 78 for gathering diagnostic information from thecomponents of the receiver such as, shown in FIG. 2A-2C. This diagnosticinformation can then be read by computer 21 to diagnose problems withthe receiver components.

What is claimed is:
 1. An antenna for use in an article surveillancesystem comprising: first and second loops arranged in a given plane,said second loop being situated entirely within said first loop so as tobe in nested relationship with said first loop, said first loop having afirst section and second and third sections extending from opposing endsof said first section of said first loop, said second loop having afirst section and second and third sections extending from opposing endsof said first section of said second loop, said first, second and thirdsections of said second loop facing the first second and third sections,respectively, of said first loop, and the spacing of the first sectionof said second loop from the first section of said first loop beinggreater than the spacing of said second and third sections of saidsecond loop from the second and third sections, respectively, of saidfirst loop; third and fourth lops arranged in said given plane, saidfourth loop being situated entirely within said third loop so as to bein nested relationship with said third loop, said third loop having afirst section and second and third sections extending from opposing endsof the first section of said third loop, said fourth loop having a firstsection and second and third sections extending from opposing ends ofsaid first section of said fourth loop, said first, second and thirdsections of said fourth loop facing said first, second and thirdsections, respectively, of said third loop, and the spacing of the firstsection of said fourth loop from the first section of said third loopbeing greater than the spacing of said second and third sections of saidfourth loop from the second and third sections respectively, of saidthird loop; said first and second loops being arranged adjacent to saidthird and fourth loops such that the first section of said first loopand the first section of said third loop are at opposing ends of saidadjacent arrangement of loops, said first through fourth loops beingconnected electrically such that said first and second loops have thesame first phase and the third and fourth loops have the same secondphase opposite said first phase, and said first and second loops andsaid third and fourth loops having relative sizes and arrangement suchthat, in combination with said nested relationship and phasing, causesaid array to exhibit substantially a single null zone along a givendirection in said plane.
 2. An antenna array in accordance with claim 1wherein: said first through fourth loops are electrically connected inseries.
 3. An antenna array in accordance with claim 1 wherein: each ofthe first, second, third and fourth loops has a fourth loop sectionconnecting further opposing ends of the second and third loop sectionsof its respective loop, said fourth loop sections of said first, second,third and fourth loops being adjacently arranged and slanted at an anglewith respect to a further given direction in said plane.
 4. An antennaarray in accordance with claim 3 wherein: the angles at which saidadjacent fourth loop sections of sad first, second, third and fourthloops are slanted with respect to the further given direction are thesame.
 5. An antenna array in accordance with claim 1 wherein: the centerof said first loop is off-set from the center of said second loop; andthe center of said third loop is off-set from the center of said fourthloop.
 6. An antenna array in accordance with claim 1 wherein: said firstand second loops are mirror images of said third and fourth loops.
 7. Atransmitter antenna array comprising: a first loop situated in a givenplane, said first loop having a maximum diameter which is greater thanany other diameter of said first loop and which is along a firstdirection in said given plane, a second loop situated in said givenplane, said second loop having a maximum diameter which is greater thanthe value of any other diameter of said second loop and which is along asecond direction in said given plane, said second loop being concentricwith and situated entirely within said first loop so as to be in nestedrelationship with said first loop and said second loop being rotatedrelative to said first loop so that said maximum diameter of said secondloop in said second direction is rotated relative to said maximumdiameter of said first loop in said first direction.
 8. An antenna inaccordance with claim 7 wherein: the angle between the first and seconddirections of said first and second loops is approximately 45 degrees.